1. Field of the Invention
The present invention relates generally to optical receivers, and more particularly, to an optical receiver that adjusts the output swing in response to a change in the signal input, with high sensitivity and over a wide dynamic range.
2. Description of the Related Art
FIG. 1 shows a conventional optical receiver used in an optical transmission system for a main line having a maximum transmission distance of 100 km, for example. A receiving element (for example, an avalanche photodiode APD) receives an input optical signal, and outputs an optical ON/OFF rectangular pattern (corresponding to the binary signals xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d, respectkvely) for every time period xcex94t, at a data rate of 1 bit. FIGS. 2(a)-2(c) illustrate a decision operation of the optical receiver for 6 bits of the input optical signal.
In the APD, an electron-hole pair induces avalanche multiplication, as caused by the optical signal, to amplify the signal current (FIG. 2(a)). This amplification makes it possible to detect even a weak optical signal. The current signal output from the APD is converted to a voltage signal VPRE by a preamplifier PRE (FIG. 2(b)). This signal VPRE is further amplified by an automatic-gain-control amplifier AGC (also known as a xe2x80x9cgain-controllable amplifierxe2x80x9d), and the resulting signal VAGC is input to a decision circuit DEC (FIG. 2(c)).
Even if the intensity of the input optical signal is weak (for example, as illustrated by the broken curve in FIG. 2(a)), the swing of the output signal VAGC of the AGC amplifier is held at a substantially constant level by the automatic gain control function of the AGC amplifier. As a result, the decision made by the decision circuit is stabilized.
In the decision circuit DEC, the datq are output in synchronism with a clock signal CLK by deciding that the output signal VAGC of the AGC amplifier is high (xe2x80x9c1xe2x80x9d) if the output signal VAGC is higher than a reference voltage VTH (also called VTH(DEC) below), or low (xe2x80x9c0xe2x80x9d) if lower than the reference voltage VTH. The clock signal CLK may be prepared from the output signal VAGC by a clock-extraction circuit CEXT.
An example of such an AGC amplifier is disclosed on pp. 815-822 of IEEE Journal0of Solid-State Circuits, Vol. 29, No. 7 (1994), and illustrated in the present FIG. 3. A voltage signal VPRE from the preamplifier PRE is amplified by three stages of amplifiers, including gain-controllable amplifiers A1 and A2, and a constant-gain amplifier A3. Then, the output of the constant-gain amplifier A3, which has a swing VA3, is input to a peak detector PD. This peak detector PD generates a DC voltage V3 based upon the swing VA3 using a capacitor CPD.
A reference circuit REF generates the nominal value of the peak voltage VN (the xe2x80x9cnominal voltage swingxe2x80x9d). This nominal value VN is adjusted by a variable voltage source P connected to the reference circuit REF.
In the gain control circuit GC, the voltage V3 and the nominal value VN are compared so that voltages VGC1 and VGC2 for controlling the gains of the amplifiers A1 and A2 are set according to the difference (i.e., the deviation of the signal swing from the nominal voltage swing). Thus, the output swing of VAGC from the swing change of the input signal VPRE is suppressed to the end of obtaining a uniform swing according to the difference in circuit construction between amplifiers A1 and A2.
According to the AGC amplifier of FIG. 3, a voltage signal VAGC having a swing of 500 mV is output at a rate of 13 Gb/sec for an input signal having a swing of either 10 mV or 300 mV. These output signals are output from two output buffers OB so as to be input to the decision circuit DEC and the clock-extraction circuit respectively.
The AGC amplifier of FIG. 3 is further equipped with an offset-control circuit OC (having external capacitors COC) for controlling the offsets of the amplifiers A1, A2, and A3.
In response to the output signal from the AGC amplifier, the decision circuit DEC decides whether the signal VAGC is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, by employing a voltage between the upper and lower ends of the swing voltage of the output signal as the reference voltage VTH(DEC).
FIG. 4 illustrates how the optical signal, as transmitted from the optical transmission passage (optical fibers, for example), is converted to a voltage signal by the preamplifier PRE. The abscissa indicates the signal voltage value, and the ordinate indicates the frequency with which the signal is generated at a predetermined voltage level. When the signals are transferred in response to the ON/OFF of the optical signal, the distribution is made, as illustrated by the solid curve in FIG. 4, with noise produced in the photoelectric converting element and in the preamplifier PRE. At this time, the reference voltage set by the decision circuit DEC is set to an intermediate value Vth(A) between the average voltage value V1 of a high signal (xe2x80x9c1xe2x80x9d) and average value V0 of a low signal (xe2x80x9c0xe2x80x9d).
The xe2x80x9c1xe2x80x9d signal, however, may have a wider voltage distribution than that of the xe2x80x9c0xe2x80x9d signal. Specifically, the voltage distribution of a xe2x80x9c1xe2x80x9d, signal takes the shape (illustrated by a broken curve in FIG. 4) in which the solid curve of FIG. 4 is depressed in the ordinate direction. This shape is especially prominent when either an APD or an erbium-doped fiber amplifier (EDFA) in combination with a PIN junction photodetector (or PIN diode) is used as the optical receiving element. See xe2x80x9cOptical Fiber Communication Techniquexe2x80x9d, published by NIKKAN KOGYO SHINBUN SHA. Voltages signals that do not exceed the reference voltage Vth(A) are decided to be xe2x80x9c0xe2x80x9d signals even if they are xe2x80x9c1xe2x80x9d signals in fact, due to this error. The probability for these erroneous decisions is the xe2x80x9cerror ratexe2x80x9d of the decision circuit. For the reference voltage Vth(A), the error rate is defined as the ratio of the area of the region under the broken-line curve to the left of Vth(A), to the entire area under the broken-line curve, which represents the voltage distribution of the actual xe2x80x9c1xe2x80x9d signal.
The error rate cannot be zero when the voltage distribution curve (broken curve) of the xe2x80x9c1xe2x80x9d, signals and the voltage distribution curve (solid curve) of the xe2x80x9c0xe2x80x9d signals overlap. However, the error rate can be reduced when the threshold voltage is considerably shifted toward V0. For example, a threshold voltage Vth(B) can be set at a point where the voltage distribution curve of the xe2x80x9c1xe2x80x9d signal and the voltage distribution curve of the xe2x80x9c0xe2x80x9d signal intersect. Hence, although not eliminated, the error rate can be reduced as low as the area ratio shown hatched in FIG. 4.
Although some actual xe2x80x9c0xe2x80x9d signals (having an intensity over Vth(B)) are detected as xe2x80x9c1xe2x80x9d signals in the decision circuit due to an error component under Vth(B), the total error rate is reduced compared with that under Vth(A). Moreover, the distribution of the signal voltage extends wider than the magnitude (V1xe2x88x92V0) of the signal anyway, so that the error rate cannot be completely reduced to 0 irrespective of where the reference voltage is set. In the optical receiver, therefore, the reference voltage is set within a range that allows and achieves a remarkably low constant error rate (e.g., 10xe2x88x9212). Then, the transmission errors that do occur can be detected and corrected with an error correction code.
FIG. 5 schematically illustrates the relationship between the set range of the reference voltage and the optical signal intensity, with an error rate considered with an APD or the combination of an EDFA and PIN diode as the optical receiving element. The xe2x80x9c1xe2x80x9d, signal indicates a wider voltage distribution than that of the xe2x80x9c0xe2x80x9d signal, as described above, so that the set range of the reference voltage is offset toward the xe2x80x9c0xe2x80x9d signal peak. When the optical signal intensity becomes lower, the set range of the reference voltage grows gradually narrower until the desired error rate cannot be achieved at all. The optical signal intensity at this point (indicated by Pmin0) is called the xe2x80x9cminimum receiver sensitivityxe2x80x9d. Pmin0 represents the point below which the overlap of the xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d curves (which represents the error rate) is too great for the desired error rate to be achieved.
To cover applications for various transmission distances, a wide input dynamic range is required, even for a receiver for a main line. When the transmission distance is short, the optical signal attenuation through the optical fibers is so low that an intense optical signal is input to the receiver. When the transmission distance is long, on the other hand, the input optical signal is very weak. Thus, the optical receiver must be capable of handling both cases.
When a strong optical signal is input to a receiver having the construction shown in FIG. 1, the following problem, discussed with additional reference to FIGS. 6(a)-6(c), is caused by the slew rate of the preamplifier. FIGS. 6(a)-6(c) illustrate response waveforms for the receiver when a strong optical signal is input. When the intensity of the optical signal is high, the output current of the optical receiving element rises, to increasing the output swing of the preamplifier. When the output swing of the preamplifier becomes excessively large, however, a signal change occurs before the output signal reaches a steady value, as illustrated, so that the output waveform partially takes a triangular shape. The partial triangular shape results because the changing rate of the output VPRE of the preamplifier has an upper limit (dV/dt) called the xe2x80x9cslew ratexe2x80x9d. If the slew rate of the AGC amplifier is sufficiently higher than that of the preamplifier, the output signal of the AGC amplifier has a similar waveform as that of the VPRE.
In the illustrated example, the first bit (left side) or the fifth bit (second bit from right side) are erroneously decided no matter where the reference voltage VTH(DEC) might be set, because the preferable VTH(DEC) for deciding the first bit as xe2x80x9c0xe2x80x9d cannot but be too high for the fifth bit to be decided as xe2x80x9c1xe2x80x9d, and the preferable VTH(DEC) for deciding the fifth bit as xe2x80x9c1xe2x80x9d cannot but be too low for the first bit to be decided as xe2x80x9c0xe2x80x9d, Vth(A). Since the output VPRE of the preamplifier has a large swing, moreover, the voltage to be applied to a transistor of the output stage of the preamplifier rises so high as to make it necesasary to use a transistor with a high breakdown voltage. Since the action rate of a transistor having a high breakdown voltage is generally lower than that of one having a low breakdown voltage, however, another problem arises in that the transistor cannot be used in a high-speed optical receiver. Thus, a highly intense optical signal is not easily handled by the construction shown in FIG. 1.
An optical receiver that is capable of avoiding these problems caused by the slew rate is disclosed on pp. 991-997 of IEEE Journal of Solid-State Circuits, Vol. 30, No. 9 (1995). FIG. 7 illustrates a construction of an optical receiver disclosed in this article. This circuit is employed in a local area network (LAN), for example, and has a transmission distance as short as several Km at most. Therefore, the APD is replaced by a PIN photodiode. The AGC amplifier is also replaced, by a limiting amplifier LA.
Although the output signal swing is held constant in the AGC amplifier by controlling the gain, the gain of the limiting amplifier LA is set at a remarkably high value so that the output signal is clamped to a predetermined voltage range, to limit the swing whenever the output swing rises above a desired value. Specifically, the reference voltage VTH(LA) is controlled by the reference voltage control circuit VCNT to be just intermediate between the high voltage (corresponding to xe2x80x9c1xe2x80x9d, for example) and the low voltage (corresponding to xe2x80x9c0xe2x80x9d, for example) of the input signal. The input signal and VTH(LA) are compared so that a constant voltage VLA(1) is output irrespective of the swing of the input signal if the input signal is higher than VTH(LA), but a constant voltage VLA(0) is output irrespective of the input signal swing if the input signal is lower than VTH(LA).
In this example, the gain of the limiting amplifier LA is set at about 60 dB so that an output signal of constant swing is achieved over a wide range of input signals, from several mV to about 1 V. With this construction, the gain of the limiting amplifier is so high that the gain (i.e., the ratio between the output voltage and the input current, also called the xe2x80x9ctransimpedancexe2x80x9d) of the preamplifier need not be raised so high. Even when an optical signal of high intensity is input, therefore, the swing of the output VPRE of the preamplifier is not so large as that of the aforementioned construction, so that the signal is neither distorted nor becomes a triangular wave, as illustrated in FIG. 6(b). This raises no problem due to the slew rate.
In this, however, if an APD or the combination of an EDFA and PIN diode are used as the optical receiving element instead of the PIN photodiode alone, the minimum receiver sensitivity is deteriorated. FIG. 8 illustrates a set range of the reference voltage VTH(LA) of the limiting amplifier LA in the preamplifier output VPRE. Because an optical receiving element having an amplifying function is used, the xe2x80x9c1xe2x80x9d signal indicates a wider voltage distribution than that of the xe2x80x9c0xe2x80x9d signal, and the set range of the reference voltage is offset towards the xe2x80x9c0xe2x80x9d signal side, as before. The output (VLA) of the limiting amplifier LA is derived by extracting a signal represented by VTH(LA)xc2x1several mV from the VPRE and by amplifying it (a voltage width of several mV being indicated by a double curve). The VTH(LA) is set just intermediate between the VPRE(0) and the VPRE(1), such that the VTH(LA) deviates from the set range of the reference value for a region where the optical signal intensity is Pmin1 or less. As a result, the minimum receiver sensitivity is at Pmin1, which is a lower sensitivity (i.e., Pmin1 is greater than than Pmin0) than that of the construction of FIG. 1.
The deterioration of the minimum receiver sensitivity does not take place in the construction of this circuit, however, because the PIN diode is used as the optical receiving element. Since the PIN diode has no amplifying action, the xe2x80x9c1xe2x80x9d signal exhibits the same voltage distribution as that of the xe2x80x9c0xe2x80x9d signal, so that the aforementioned problem does not occur even if the VTH(LA) is set just intermediate between the VPRE(0) and the VPRE(1).
Thus, to avoid the problem, the reference voltage VTH(LA) of the limiting amplifier LA can be adjusted, not to the center of the VPRE, but towards the side of the xe2x80x9c0xe2x80x9d level. However, this method is extremely difficult to realize, because the intensity of the optical signal to be adjusted is weak (especially within a range less than Pmin1), and because the reference voltage VTH(LA) must be precisely adjusted under the condition of a small swing of the VPRE. Since the intensity of the optical signal fluctuates by the degradation in the characteristics of the transmitter or the optical fibers, it is necessary to periodically readjust the reference voltage VTH(LA). Thus, it has been impossible to use a wide dynamic range receiver in a LAN, as disclosed in this article, as has been used for long distance transmission.
Other techniques for widening the dynamic range of the optical receiver are disclosed in Japanese Patent Laid Open No. 93-259752 (Tokukai-Hei 05-259752), which discloses an improved technique for setting a reference potential of the discrimination (decision) circuit, according to which the circuit decides a voltage signal input thereto to be either xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d, and in Japanese Patent Laid Open No. 96-139526 (Tokukai-Hei 08-139526), No. 95-193437 (Tokukai-Hei 07-193437), No. 95-38342 (Tokukai-Hei 07-38342), and No. 93-67930 (Tokukai-Hei 05-67930), which disclose improved circuitries for setting the gain of the preamplifier. All of these techniques are aimed to be applied for optical communication systems having a transmission rate of 100 MHz. Therefore, these techniques are expected to have sufficient responses to receive optical pulses at intervals of 10 ns, but cannot be expected to respond to optical pulses at intervals of 100 ps, as is the case for optical communication systems for trunk lines having a transmission rate of 10 GHz.
Japanese Patent Laid Open No. 97-246879 (Tokukai-Hei 09-246879), and No. 93-218758 (Tokukai-Hei 05-218758) disclose techniques for suppressing deterioration of the width of the voltage pulse in the optical receiver, but the key techniques for solving the problems illustrated in FIGS. 1 and 7, discussed above, are not disclosed.
An object of the present invention is to provide an optical receiver having a high sensitivity and a wide dynamic range.
To achieve this and other objects, the invention provides an optical receiver for converting an optical signal into an electrical signal, including an optical receiving element (photoelectric converter) for converting the optical signal into a current signal, a preamplifier for converting the current signal output by the photoelectric converter into a voltage signal, an amplifier having a limiting function for linearly amplifying the voltage signal when the difference between the voltage signal and a reference voltage is smaller than a predetermined voltage, and for limitedly amplifying the voltage signal when the difference is larger than the predetermined voltage, and an automatic-gain-control amplifier for amplifying the output of the amplifier having the limiting function, to produce a signal having a constant swing. The amplifier having the limiting function and the circuit stage performing its function are referred to below as the xe2x80x9cpresent limiting amplifierxe2x80x9d and the xe2x80x9cpresent limiting amplification stagexe2x80x9d, respectively.
The predetermined voltage at which the present limiting amplifier starts its limited amplification is desirably set larger than the difference between the output signal of the preamplifier in the minimum receiver sensitivity and the reference voltage. The output signal by the preamplifier in the minimum receiver sensitivity is determined at a desired error rate by the photoelectric converter and the preamplifier.
The output signal swing of the amplifier having the limiting function is desirably restricted to satisfy the following formula within a desired dynamic range of the optical signal:
xcex94VAWL less than Rsxc2x7xcex94txe2x80x83xe2x80x83(Formula 1) 
In Formula 1, xcex94VAWL is the output signal swing of the amplifier having the limiting function, xcex94t is the time period to be occupied by an optical signal representing 1 bit, and Rs is the slew rate of the amplifier having the limiting function.
The photoelectric converter is preferably constructed of an avalanche photodiode APD or an erbium-doped fiber amplifier EDFA and a PIN photodiode in combination, so that the minimum receiver sensitivity can be improved.
When the optical signal intensity is weak, the amplifier with the limiting function linearly amplifies the voltage signal VPRE according to the optical signal. The AGC amplifier disposed at the next stage thereto amplifies the output AWL of the amplifier with the limiting function, and the AGC amplifier provides its output voltage signal VAGC to the decision circuit. The decision circuit sets its reference voltage to decide whether the input signal VAGC is xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d, corresponding to the signal swing of the VAGC (the voltage difference between the signals of xe2x80x9c1xe2x80x9d and xe2x80x9c1xe2x80x9d). Thus, the linear amplifications of the voltage signal by the amplifier with the limiting function and the AGC amplifier enable the decision circuit to set its reference voltage to an optimum point, even if the optical receiver accepts fairly weak optical signals. When the optical signal intensity is high, on the other hand, the amplifier with the limiting function acts to limit the output signal swing so as to prevent the output signal of the preamplifier from distorting and to suppress an increase of the error rate.
In a preferred embodiment of the invention, an optical transmission system terminal unit or an optical transmission system includes, in combination, an optical receiver including an optical receiving element (photoelectric converter), a first amplifier (which may be a preamplifier) for converting a current signal output from the optical receiving element into a voltage signal, a second amplifier (which may be a gain-controllable amplifier or an AGC amplifier) for reducing the swing dispersion of the voltage signal output from the first amplifier, a decision circuit for processing (or deciding) the voltage signal output from the second amplifier, and a third amplifier disposed between the first amplifier and the second amplifier. Specifically, the following functions are required for the third amplifier mentioned above. One function is to set its output signal swing (like xcex94VAWL in FIG. 9) smaller than the product of the slew rate of the third amplifier and the time period occupied by one bit of an optical signal.
Another function required for the third amplifier is to amplify the voltage signal input from the first amplifier thereto when the voltage signal has a smaller swing than a predetermined voltage swing. This function is important for detecting the input signal of an especially minute swing. As a result, the linearly amplified range of the third amplifier is set to be wider than that of the limiting amplifier of the prior art. Generally, the gain drops if the linear amplification range is extended, but the third amplifier is utilized in combination with the second amplifier at a downstream stage, so that no practical problem arises.
According to the present invention, when the optical signal intensity is weak, the amplifier with the limiting function acts as a linear amplification so that the reference value of the decision circuit can be set to the optimum point by executing a further linear amplification in the AGC amplifier at the downstream stage. When the optical signal intensity is strong, the amplifier with the limiting function performs a limiting action to prevent the output signal swing of the preamplifier from being distorted, which would increase the error rate. As a result, the optical receiver has a high sensitivity and a wide dynamic range.